Tetrahedron Letters No. 51, pages 4507-4508, 1970 describes the reaction of 3-alken-1-ols with aldehydes and their use for producing the aroma chemicals rose oxide and dihydrorose oxide. Also mentioned here is the reaction of 3-methylbutanal with isoprenol under acidic conditions.
SU 825 528 discloses a process for the preparation of di- and tetrahydropyrans and tetrahydropyranols by reacting 2-methylbuten-1-ol-4 (isoprenol) with aldehydes or ketones in the presence of an acidic catalyst, where the acidic catalyst is used in an amount of from 0.0001 to 0.01% by weight, based on the amount of isoprenol, and the reaction is carried out at a temperature of from 0 to 25° C. in an organic solvent. The catalysts specified are the ion exchange resin KU-2 (sulfonated polystyrene resin), para-toluene sulfonic acid, sulfuric acid or perchloric acid. By way of example, the reaction of isoprenol with isobutyraldehyde in the presence of KU-2, inter alia, is described.
EP 1 493 737 A1 discloses a process for the preparation of mixtures of ethylenically unsaturated 4-methyl- and 4-methylenepyrans and the corresponding hydroxypyrans by reacting the corresponding aldehydes with isoprenol, where the reaction is initiated in a reaction system in which the molar ratio of aldehyde to isoprenol is greater than 1, i.e. the aldehyde is used in excess. Moreover, the document discloses the subsequent dehydrogenation of said mixtures to give the desired ethylenically unsaturated pyrans.
Suitable catalysts specified for the first reaction step are mineral acids, such as hydrochloric acid or sulfuric acid, but preferably methanesulfonic acid or para-toluene sulfonic acid.
JP 2007-154069 relates to 2-substituted 4-hydroxytetrahydropyrans with a content of the cis-diastereomer of from 70 to 95% by weight. Moreover, the document discloses a process for the preparation of same, by reacting isoprenol with a corresponding aldehyde in the presence of an aqueous solution of an acidic catalyst. Here, the reaction has to be carried out at a concentration of the aqueous catalyst solution either in the range from 1:10% by weight at a temperature of from 0 to 100° C., or in the region of 10% by weight or above at a temperature of from 0 to 30° C. The possible acidic catalysts mentioned are generally also ion exchange resins.